Temporarily force kCanvasSharedBitmapToSharedImage to false for Blink unittests.

Allocating a test Shared Image Interface is needed for
CanvasResourceProviderSharedBitmap when enabling the
kCanvasSharedBitmapToSharedImage feature in the software rendering
mode. In preparation for enabling this feature, temporarily force
kCanvasSharedBitmapToSharedImage to false for the Blink unittests until
the test shared image interface is handled in the unittests.

Bug:40064122

Change-Id: I34d797cd5e7aa1063d7bcdd2f437502e2032a824
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5798982
Reviewed-by: Aaron Krajeski <aaronhk@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Maggie Chen <magchen@chromium.org>
Auto-Submit: Maggie Chen <magchen@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1345688}